1. Field of the Invention
The present invention relates to a printing control apparatus of a full line system, in which the same pixel can be printed by the use of a plurality of print elements, and a printing control method.
2. Description of the Related Art
One type of printing apparatus is, for example, an ink jet printing apparatus using an ink jet print head provided with nozzles constituting print elements. Such an ink jet printing apparatus uses an ink jet print head having a plurality of nozzles, which are formed along a nozzle array and can eject ink, for printing an image on a print medium. Such an ink jet printing apparatus is of a serial system or a full line system. In the case of the serial system, an operation for ejecting ink from nozzles while moving a print head in a main scan direction crossing a nozzle array and an operation for conveying a print medium in a sub scan direction crossing the main scan direction are repeated so as to print an image. In contrast, the full line system uses an elongated print head having a nozzle array extending over the entire print area in a width direction of a print medium, and then, sequentially conveys a print medium in a direction crossing the nozzle array while ejecting ink from nozzles of the print head so as to print an image.
The printing apparatus of the serial system adopts a multi-pass print system in which a print head can scan a print area a plurality of times, and then, print an image on a print medium, so as to form ink dots on one line along a main scan direction by using a plurality of nozzles formed at the print head. In this manner, it is possible to suppress deviation (partial) in frequency of use of a nozzle and an influence of variations of ink ejection characteristics of each of nozzles to a low level.
In contrast, since the relationship between the orientation of the nozzle array of the print head and the widthwise position of the print medium is fixed in the printing apparatus of the full line system (i.e., a line printer), a plurality of nozzles cannot form ink dots on one line along the width of the print medium. Consequently, there is a fear of occurrence of the deviation in frequency of use of the nozzle. In particular, in a case where dithering is adopted as a pseudo-halftone representing method, a dither matrix is repeatedly used to print an image, thereby making the deviation in frequency of use of the nozzle conspicuous. On the other hand, in a case where error diffusion is adopted as the pseudo-halftone representing method, an accidental error value is distributed and diffused to a pixel that has not yet processed, and therefore, an output result after the error diffusion cannot have a regular pattern but has a random pattern, thereby reducing the deviation in frequency of use of the nozzle.
However, an error generated in a target pixel is weighted and diffused to peripheral pixels, and therefore, the error diffusion requires much processing time. In view of this, a line printer, for which higher-speed printing is required, adopts dithering as the pseudo-halftone representing method from the viewpoint of a processing speed while requiring the suppression of the deviation in frequency of use of the nozzle to a low level. In the case of the marked deviation in frequency of use of the nozzle, a nozzle of higher frequency of use reaches the end of its lifetime earlier than a nozzle of lower frequency of use, resulting in a short lifetime of the print head.
Japanese Patent Laid-Open No. 2009-220304 proposes a method in which a plurality of print heads extending in a width direction of a print medium are used in a line printer, and then, nozzles of different print heads form adjacent dots on one line in the width direction of the print medium.
However, the technique disclosed in Japanese Patent Laid-Open No. 2009-220304 needs to produce a dot forming pattern per print head in consideration of a constraint of arrangement of dots to be formed by a plurality of print heads in a case where there are a predetermined number or more of dots to be formed adjacently in the width direction of the print medium. In addition, the load of data processing becomes considerably heavy. Particularly, the line printer performs printing at a high speed, and therefore, the need of such processing possibly induces an increase in size of a control circuit or an increase in cost.
Moreover, in a case where the print heads to be used are switched in sequence such that dots formed by the same print head are not continuous in the width direction of the print medium, there may possibly raise a fear of an interference between image data and a switch pattern of the print head to be used. For example, such an interference occurs in a case where four print heads are used to form a dot pattern in the following manner: a one-dot formation position, a one-dot non-formation position, a one-dot non-formation position, and a one-dot non-formation position are sequentially arranged on one line in the width direction of the print medium corresponding to the direction of the nozzle array. In the case of the repetition of the above-described regular dot pattern, variations of ink ejection characteristics of the four print heads more markedly influence a print image than in the case of a random dot pattern.
Additionally, dots may not be sequentially formed in most cases on an image represented with, principally, a halftone, such as a photographic image. In this manner, in a case where there are few dots to be sequentially formed, the plurality of print heads hardly share the load of forming sequential dots in the technique disclosed in Japanese Patent Laid-Open No. 2009-220304. Consequently, a single print head has nozzles of a high frequency of use, thereby raising a fear of impairing prolongation of a lifetime of the print head.